Fixes DOC-3360 to make help context-sensitive

themes/edx.org/lms/templates/dashboard.html

 <%page expression_filter="h"/>
 <%inherit file="main.html" />
+<%def name="online_help_token()"><% return "learnerdashboard" %></%def>
 <%namespace name='static' file='static_content.html'/>
 <%!
 from django.utils.translation import ugettext as _

themes/edx.org/lms/templates/header.html

 ## mako
-<%page expression_filter="h"/>
+<%page expression_filter="h" args="online_help_token"/>
 <%namespace name='static' file='static_content.html'/>
 <%namespace file='main.html' import="login_query"/>
 <%!